RET antennas are now widely used as based station antennas in cellular communications systems. Prior to the introduction of RET antennas, when the coverage area for a conventional base station antenna needed to be adjusted, it was necessary for a technician to climb the antenna tower on which the antenna was mounted and manually adjust the pointing angle of the antenna. Typically, the coverage area of the antenna is adjusted by changing the so-called “tilt” angle of the antenna, which is the angle in the elevation plane of the boresight pointing direction of the antenna beam generated by the antenna. The introduction of RET antennas allowed a cellular operator to electrically adjust the tilt angle of the antenna beam by sending a control signal to the antenna.
Base station antennas are typically implemented as phased array antennas that include an array of radiating elements. The arrays are often linear arrays where the radiating elements are stacked along a vertical axis that is perpendicular the plane defined by the horizon, although planar arrays and arrays having other shapes may also be used. A radio frequency (RF) signal that is to be transmitted by a phased array antenna may be divided into a plurality of sub-components, and each sub-component may be transmitted through a respective sub-set of the radiating elements that is typically referred to as a “sub-array.” In some cases, each sub-array may include a single radiating element, while in other cases some or all of the sub-arrays may include two or more radiating elements that each transmit the same sub-component of the RF signal. The magnitudes of the sub-components of the RF signal may be the same or different, and the relative phases of the sub-components of the RF signal may be set so that the antenna beam formed by the array has a desired shape. In many cases, the relative phases of the sub-components of the RF signal are set by designing the sub-components of the RF signals to traverse respective paths through the antenna having different lengths, where the differences in path lengths provide desired phase shifts that electrically shape the antenna beam in a desired fashion.
A RET antenna further includes a RET system that allows a cellular operator to dynamically adjust the tilt angle of the antenna beam. In particular, the RET system allows the cellular operator to add additional phase shifts to the sub-components of the RF signals that are to be transmitted (and received) by the antenna, which changes the tilt angle of the antenna beam generated by the antenna. The RET system typically comprises a drive motor, a transmission mechanism, and a phase shifter for each array of radiating elements. When cross-polarized radiating elements are used, the RET system may include a single drive motor and transmission mechanism per array, but two phase shifters are provided to adjust the phases of the sub-components of the RF signals having the two respective polarizations. Each phase shifter may include a fixed element, a moveable element and a phase shifter driving device. The phase shifter driving device may convert a motion generated by the drive motor and transmitted via the transmission mechanism into a movement of the moveable element of the phase shifter relative to the fixed element so as to change a phase of a signal, thereby realizing the adjustment of the electric tilt angle.
A number of different types of phase shifters are known in the art, including, for example, rotary wiper arm phase shifters, trombone style phase shifters and sliding dielectric phase shifters. In a rotary wiper arm phase shifter, a wiper printed circuit board is mounted above a main printed circuit board by a pivot pin so that the wiper printed circuit board may rotate above the main printed circuit board. Typically the phase shifter will include one or more power dividers that split an RF signal that is input to the phase shifter into a plurality of sub-components. At least a portion of the RF signal is transferred to the wiper printed circuit board and then coupled from the wiper printed circuit board to a transmission path on the main printed circuit board. The path length through the phase shifter of each sub-component of the RF signal that is transferred to the wiper printed circuit board depends upon the position of the wiper printed circuit board above the main printed circuit board. Thus, by moving the wiper printed circuit board (e.g., using an actuator) the phases of the sub-components of the RF signal may be adjusted in order to change the tilt angle of the antenna beam. Trombone style phase shifters operate in a similar manner, except that the moveable element of the phase shifter moves linearly instead of along an arc. Sliding dielectric phase shifters have a fixed path length, but move dielectric material that is part of the RF transmission lines through the phase shifter in order to change the dielectric constant of transmission line substrate, which acts to change the phase shift.
Many modern base station antennas include multiple arrays of radiating elements. The tilt angle for the antenna beam generated by each array is typically independently adjusted. Accordingly, in order to realize different electric tilt angles for the different arrays, it is usually necessary to adjust the respective phase shifters in different directions and by different amounts. As noted above, each array will typically have an associated drive motor, transmission mechanism and phase shifter, which can take up a significant amount of room within the interior of the antenna cavity. In addition, the space of the antenna cavity may be narrow and the routing may be complicated, which makes the available space extremely limited.